Method and system for beam expansion in a display device

ABSTRACT

An exit pupil extender wherein the relative amount of different color components in the exit beam is more consistent with that of the input beam. In order to compensate for the uneven amount in the diffracted color components in the exit beam, the exit pupil extender, comprises a plurality of layers having additional diffraction gratings so as to increase the amount of diffracted light for those color components with a lower amount. Additionally, color filters disposed between layers to reduce the diffracted light components with a higher amount.

FIELD OF THE INVENTION

The present invention relates generally to a display device and, morespecifically, to a display that uses one or more diffractive elementsfor extending the exit pupil of the display for viewing.

BACKGROUND OF THE INVENTION

While it is a common practice to use a low-resolution liquid-crystaldisplay (LCD) panel to display network information and text messages ina mobile device, it is preferred to use a high-resolution display tobrowse rich information content of text and images. A microdisplay-basedsystem can provide full color pixels at 50–100 lines per mm. Suchhigh-resolution is generally suitable for a virtual display. A virtualdisplay typically consists of a microdisplay to provide an image and anoptical arrangement for manipulating light emerging from the image insuch a way that it is perceived as large as a direct view display panel.A virtual display can be monocular or binocular.

The size of the beam of light emerging from imaging optics toward theeye is called exit pupil. In a Near-Eye Display (NED), the exit pupil istypically of the order of 10 mm in diameter. Further enlarging the exitpupil makes using the virtual display significantly easier, because thedevice can be put at a distance from the eye. Thus, such a display nolonger qualifies as an NED, for obvious reasons. Head-Up Displays are anexample of the virtual display with a sufficiently large exit pupil.

WO 99/52002 discloses a method of enlarging the exit pupil of a virtualdisplay. The disclosed method uses three successive holographic opticalelements (HOEs) to enlarge the exit pupil. In particular, the HOEs arediffractive grating elements arranged on a planar, light transmissivesubstrate 6, as shown in FIG. 1. As shown, light from an image source 2is incident upon the first HOE, or H1, which is disposed on one side ofthe substrate 6. Light from H1, coupled to the substrate 6, is directedtoward the second HOE, or H2, where the distribution of light isexpanded in one direction. H2 also redirects the expanded lightdistribution to the third HOE, or H3, where the light distribution isfurther expanded in another direction. The holographic elements can beon any side of the substrate 6. H3 also redirects the expanded lightdistribution outward from the substrate surface on which H3 is disposed.The optical system, as shown in FIG. 1, operates as a beam-expandingdevice, which maintains the general direction of the light beam. Such adevice is also referred to as an exit pupil extender (EPE).

The EPE, such as that shown in FIG. 1, results in color non-uniformity,thereby degrading the quality of the reproduced virtual image. The colornon-uniformity is mainly due to the fact that light beams of differentcolors travel different paths in the substrate 6, as shown in FIG. 2.For illustration purposes, only two colors, represented by λ₁ and λ₂,are used to show the source of color non-uniformity in the prior artEPE, with λ₁<λ₂.

In FIG. 2, only two HOEs are used, but the source of colornon-uniformity is the same when three or more HOEs are used. The firstHOE, or H1, typically has a diffractive structure consisting of paralleldiffractive fringes for coupling incident light into the substrate 6 anddirecting the light distribution within the substrate 6 toward thesecond HOE, or H2. The substrate 6 acts as a light guide to trap thelight beams between its two surfaces mainly by means of total internalreflection (TIR). As shown in FIG. 2, the diffractive elements H1 and H2are both disposed on the lower surface of the substrate 6. In such anoptical device, TIR is complete only at the upper surface, because partof the light is diffracted out from the lower surface of the substratetoward the viewer's eye.

It is known that the diffraction angle inside the substrate 6 isgoverned by:sin(θ_(i))−n sin(θ_(m))=mλ/dwhere

-   -   d is the grating period of the diffractive element (here H1)    -   λ is the wavelength    -   n is the refractive index of the substrate    -   m is the diffraction order    -   θ_(i) is the angle of incident, and    -   θ_(m) is the angle of diffraction in m^(th) order.        As can be seen from Equation 1, the diffraction angle θ_(m)        increases with wavelength λ. Thus, the diffraction angle θ_(m1)        is smaller than the diffraction angle θ_(m2). As a result, the        interval L between two successive TIRs also varies with        wavelength. The interval L₁ for λ₁ is smaller than the interval        L₂ for λ₂. Thus, the distribution of outgoing light in the η        direction is not uniform for all wavelengths, although the        grating structure can be designed so that the output is        homogeneous for one wavelength. As can be seen in FIG. 2, the        shorter wavelength λ₁ experiences more “hits” than and λ₂ on the        diffractive elements H2. Consequently, more light of the shorter        wavelength λ₁ “leaks” out of the diffractive element H2 in the        area near H1. In a display where three primary colors (red,        green, blue) are used, an EPE of FIG. 2 will cause an uneven        color distribution of the light exiting the diffractive grating        structure of H2. Thus, the color may appear bluish on the near        end and reddish on the far end, relative to H1. As the distance        along the η direction increases, the uneven color distribution        becomes more noticeable.

It should be noted that light can “leak” out of the substrate 6 from thelower surface where H2 is located or from the upper surface. Thedistribution of outgoing light from the upper surface is similar to thatfrom the lower surface.

It is advantageous and desirable to provide a method and system forimproving the color uniformity in light distribution in an exit pupilextender.

SUMMARY OF THE INVENTION

It is an objective to the present invention to provide an exit pupilextender wherein the relative amount of different color components inthe exit beam is more consistent with that of the input beam. In orderto compensate for the uneven amount in the diffracted color componentsin the exit beam, the exit pupil extender, according to the presentinvention, comprises a plurality of layers having additional diffractiongratings so as to increase the amount of diffracted light for thosecolor components with a lower amount.

Thus, the first aspect of the present invention provides an opticaldevice comprising:

an exit surface section;

an input surface section to admit a light beam, the light beamcomprising at least a first wavelength component and a second wavelengthcomponent; and

at least a first layer and a second layer, each of the first and secondlayers comprising a first diffractive element and a second diffractiveelement, such that

at least a part of the admitted light beam is diffracted in the firstdiffractive element in the first layer for providing a first diffractedportion;

at least a part of the admitted light beam is diffracted in the firstdiffractive element in the second layer for providing a seconddiffracted portion;

at least a part of the first diffracted portion is further diffracted inthe second diffractive element in the first layer, thereby forming afirst part of an exit beam exiting through the exit surface section; and

at least a part of the second diffracted portion is further diffractedin the second diffractive element in the second layer, thereby forming asecond part of the exit beam exiting through the exit surface section.

According to the present invention, the amount of the first wavelengthcomponent in the first part of the exit beam is greater than the amountof the second wavelength component in the first part of the exit beam,and the amount of the second wavelength component in the second part ofthe exit beam is greater than the amount of the first wavelengthcomponent in the second part of the exit beam.

According to the present invention, the admitted light beam furthercomprises a third wavelength component and said optical device furthercomprises:

a third layer comprising a first diffractive element and a seconddiffractive element, such that

at least a part of the admitted light beam is diffracted in the firstdiffractive element in the third layer for providing a third diffractedportion, and

at least a part of the third diffracted portion is further diffracted inthe second diffractive element in the third layer, thereby forming athird part of the exit beam exiting through the exit surface section.

According to the present invention, the amount of the first wavelengthcomponent in the first part of the exit beam is greater than the amountof the second wavelength component in the first part of the exit beamand is also greater than the amount of the third wavelength component inthe first part of the exit beam;

the amount of the second wavelength component in the second part of theexit beam is greater than the amount of the first wavelength componentin the second part of the exit beam and is also greater than the amountof the third wavelength component in the second part of the exit beam;and

the amount of the third wavelength component in the third part of theexit beam is greater than the amount of the first wavelength componentin the third part of the exit beam and is also greater than the amountof the second wavelength component in the third part of the exit beam.

According to the present invention, the optical device further comprisesa filter disposed between the first diffractive element in the firstlayer and the first diffractive element in the second layer so as toreduce the amount of the first wavelength component in the second partof the exit beam.

According to the present invention, the optical device further comprisesanother filter disposed between the first diffractive element of thesecond layer and the first diffractive element of the third layer so asto reduce the amount of the second wavelength component in the secondpart of the exit beam and the amount of the second wavelength componentin the third part of the exit beam.

According to the present invention, the first and second diffractiveelements in the first, second and third layers can holographic opticalelements or diffractive optical elements mechanically or chemicallyproduced.

According to the present invention, the first wavelength component has afirst wavelength range, the second wavelength component has a secondwavelength range longer than the first wavelength range, and the thirdwavelength component has a third wavelength range greater than thesecond wavelength range.

According to the present invention, the first wavelength componentcomprises a blue color wavelength component, the second wavelengthcomponent comprises a green color component and the third wavelengthcomponent comprises a red color component.

The second aspect of the present invention provides a method ofimproving color uniformity in an exit beam in an optical device, theoptical device having

an input surface section to admit a light beam, the light beamcomprising at least a first wavelength component and a second wavelengthcomponent; and

an exit surface section for allowing the exit beam to exit the opticaldevice through the exit surface. The method comprises:

providing at least a first layer, a second layer and a third layer inthe optical device;

providing a first diffractive element and a second diffractive elementin the first layer;

providing a first diffractive element and a second diffractive elementin the second layer,

providing a first diffractive element and a second diffractive elementin the third layer, such that

at least a part of the admitted light beam is diffracted in the firstdiffractive element in the first layer for providing a first diffractedportion;

at least a part of the admitted light beam is diffracted in the firstdiffractive element in the second layer for providing a seconddiffracted portion;

at least a part of the first diffracted portion is further diffracted inthe second diffractive element in the first layer, thereby forming afirst part of an exit beam exiting through the exit surface section; and

at least a part of the second diffracted portion is further diffractedin the second diffractive element in the second layer, thereby forming asecond part of the exit beam exiting through the exit surface section,wherein

the amount of the first wavelength component in the first part of theexit beam is greater than the amount of the second wavelength componentin the first part of the exit beam and is also greater than the amountof the third wavelength component in the first part of the exit beam;

the amount of the second wavelength component in the second part of theexit beam is greater than the amount of the first wavelength componentin the second part of the exit beam and is also greater than the amountof the third wavelength component in the second part of the exit beam;and

the amount of the third wavelength component in the third part of theexit beam is greater than the amount of the first wavelength componentin the third part of the exit beam and is also greater than the amountof the second wavelength component in the third part of the exit beam.

According to the present invention, the method further comprises:

disposing a filter between the first diffractive element in the firstlayer and the first diffractive element in the second layer so as toreduce the amount of the first wavelength component in the second partof the exit beam and the amount of the first wavelength component in thethird part of the exit beam;

disposing another filter between the first diffractive element of thesecond layer and the first diffractive element of the third layer so asto reduce the amount of the second wavelength component in the secondpart of the exit beam and the amount of the second wavelength componentin the third part of the exit beam.

The third aspect of the present invention provides a display module,comprising:

an optical engine for receiving image data;

a display device operatively connected to the optical engine for formingan image based on the image data; and

an exit pupil extender, comprising:

-   -   an exit surface section;    -   an input surface section to admit a light beam, the light beam        comprising at least a first wavelength component and a second        wavelength component; and    -   at least a first layer, a second layer and a third layer, each        of the first, second and third layers comprises a first        diffractive element and a second diffractive element, such that    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the first layer for providing a        first diffracted portion;    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the second layer for providing a        second diffracted portion;    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the third layer for providing a        third diffracted portion;    -   at least a part of the first diffracted portion is further        diffracted in the second diffractive element in the first layer,        thereby forming a first part of an exit beam exiting through the        exit surface section;    -   at least a part of the second diffracted portion is further        diffracted in the second diffractive element in the second        layer, thereby forming a second part of the exit beam exiting        through the exit surface section; and    -   at least a part of the third diffractive portion is further        diffracted in the third diffractive element in the third layer,        thereby forming a third part of the exit beam exiting through        the exit surface section, wherein    -   the amount of the first wavelength component in the first part        of the exit beam is greater than the amount of the second        wavelength component in the first part of the exit beam and is        also greater than the amount of the third wavelength component        in the first part of the exit beam;    -   the amount of the second wavelength component in the second part        of the exit beam is greater than the amount of the first        wavelength component in the second part of the exit beam and is        also greater than the amount of the third wavelength component        in the second part of the exit beam; and    -   the amount of the third wavelength component in the third part        of the exit beam is greater than the amount of the first        wavelength component in the third part of the exit beam and is        also greater than the amount of the second wavelength component        in the third part of the exit beam.

The fourth aspect of the present invention provides an electronicdevice, comprising:

a data processing unit;

an optical engine operatively connected to the data processing unit forreceiving image data from the data processing unit;

a display device operatively connected to the optical engine for formingan image based on the image data; and

an exit pupil extender, comprising:

-   -   an exit surface section;    -   an input surface section to admit a light beam, the light beam        comprising at least a first wavelength component and a second        wavelength component; and    -   at least a first layer, a second layer and a third layer, each        of the first, second and third layers comprising a first        diffractive element and a second diffractive element, such that    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the first layer for providing a        first diffracted portion;    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the second layer for providing a        second diffracted portion;    -   at least a part of the admitted light beam is diffracted in the        first diffractive element in the third layer for providing a        third diffracted portion;    -   at least a part of the first diffracted portion is further        diffracted in the second diffractive element in the first layer,        thereby forming a first part of an exit beam exiting through the        exit surface section;    -   at least a part of the second diffracted portion is further        diffracted in the second diffractive element in the second        layer, thereby forming a second part of the exit beam exiting        through the exit surface section; and    -   at least a part of the third diffractive portion is further        diffracted in the third diffractive element in the third layer,        thereby forming a third part of the exit beam exiting through        the exit surface section, wherein

the amount of the first wavelength component in the first part of theexit beam is greater than the amount of the second wavelength componentin the first part of the exit beam and is also greater than the amountof the third wavelength component in the first part of the exit beam;

the amount of the second wavelength component in the second part of theexit beam is greater than the amount of the first wavelength componentin the second part of the exit beam and is also greater than the amountof the third wavelength component in the second part of the exit beam;and

the amount of the third wavelength component in the third part of theexit beam is greater than the amount of the first wavelength componentin the third part of the exit beam and is also greater than the amountof the second wavelength component in the third part of the exit beam.

According to the present invention, the electronic device comprises aportable device, such as a mobile phone, personal digital assistant(PDA), communicator, portable Internet appliance, hand-held computer,digital video and still camera, wearable computer, computer game device,specialized bring-to-the-eye product for viewing and other portableelectronic devices. However, the exit pupil extender can also be used ina non-portable device, such as a gaming device, vending machine,bank-o-mat, home appliance, such as an oven, microwave oven and otherappliances, and other non-portable electronic devices.

The present invention will become apparent upon reading the descriptiontaken in conjunction with FIGS. 3 a to 5.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation showing an exit pupil extenderusing three diffractive elements.

FIG. 2 is a schematic representation showing a prior art exit pupilextender.

FIG. 3 a is a schematic representation showing a part of the exit pupilextender, according to the present invention.

FIG. 3 b is a schematic representation showing the diffraction of theinput beam at various layers of the exit pupil extender, according tothe present invention.

FIG. 3 c is a schematic representation showing the composition of thecolor components in the exit beam.

FIG. 3 d is a schematic representation showing another embodiment of theexit pupil extender, according to the present invention.

FIG. 4 is a schematic representation showing a different view of theexit pupil extender, according to the present invention.

FIG. 5 is a schematic representation showing a mobile device having avirtual display system.

BEST MODE TO CARRY OUT THE INVENTION

Instead of using a homogeneous substrate 6, as shown in FIG. 2, the exitpupil extender (EPE) 10, of the present invention, uses a substrate 60comprising a plurality of layers, each layer having a diffractiongrating, as shown in FIGS. 3 a–3 d.

As shown in FIG. 3 a, the substrate 60 has three layers of opticalmaterial 110, 120 and 130. The first layer 110 has an input diffractiongrating 112 and an output diffraction grating 114. Likewise, the secondlayer 120 has an input diffraction grating 122 and an output diffractiongrating 124, and the third layer 130 has an input diffraction grating132 and an output diffraction grating 134. As shown in FIG. 3 a, anincoming light beam 70 having red, green and blue components (RGB)enters the substrate 60 through the input grating 112 on the first layer110. After being diffracted in gratings 112, 122 and 132, the light beamis broadened and exits through the upper surface of the third layer 130.The exit light beam is denoted by reference numeral 80. In order toachieve an exit beam in which the relative amount of color components ismore consistent with that of the color components in the incoming beam,the grating period on the diffraction gratings on each layer is chosendifferently. The grating period, p, is calculated partly based on thedesirable field-of-view (FOV) of the beam expander as follows:p=λ(1+sin(FOV/2))

For example, if the FOV is 24 degrees and λ is 475 nm, then the periodis 393 nm. According to the present invention, the period, p₁, of thegrating pair (132, 134) is calculated based on the wavelength of theblue color component, or 475 mm. In order to allow the green colorcomponent to diffract more efficiently, the period, p₂, of the gratingpair (122, 124) is calculated based on the wavelength of the green colorcomponent (525 nm) and p₂=435 nm. Likewise, the period p₃ of the gratingpair and (112, 114) is calculated based on λ₃=630 nm, or p₃=522 nm. Dueto the differences in the diffraction efficiency for different colorcomponents in different gratings, the amount of diffracted light foreach color component in the each of the layers 110, 120 and 130 variesconsiderably. For example, in the first layer 110, the amount of thediffracted light for the blue color is greater than that for the greencolor, which is greater than that for the red color. As a result, whenthe reduced incoming light beam exits the upper surface of the firstlayer 110 and enters into the second layer 120, the remaining bluecomponent is relatively insignificant. In the second layer 120, thediffraction efficiency for the green color component is greater thanthat for the red component. As a result, when the further reducedincoming light exits the upper surface of the second layer 120 andenters into the third layer 130, the remaining green component becomesrelatively insignificant. Thus, the diffracted light in the third layer130 is mainly of the red component. As shown in FIG. 3 b, significantamount of diffracted light in red color occurs in all three layers 110,120 and 130. Significant amount of diffracted light in green coloroccurs only in the first layer 110 and the second layer 120. However, inthe first layer 100, the dominant diffracted color component is blue.

After going through one or more internal total reflections at the uppersurface of the layers and one or more diffraction processes at thediffraction gratings 134, 124, 114, the diffracted light exits the uppersurface as a broad beam 80. As shown in FIG. 3 c, the blue colorcomponent mainly comes from the first layer 110, while the green colorcomponent mainly comes from the first layer 110 and the second layer120. The red color component comes from all three layers 110, 120 and130.

In general, the relative amount of light in different color componentsin the exit beam is very different from that of the input beam. In orderto compensate for the uneven distribution in color components in theexit beam in an exit pupil extender, additional diffraction gratings areused to increase the diffracted light amount in certain colorcomponents. Furthermore, the depth or height, h, of the diffractiongrooves can be chosen in order to optimize the light coupling into thelayers. The height, h, depends on the refractive index of the layers,the wavelength, the period, p, and the shape of the diffraction grooves.

It is possible to introduce color filters into the exit pupil extender10 to adjust the color distribution in the exit beam 80. For example, itis possible to provide a yellow filter 140 between layer 120 and layer110 to block the undiffracted blue color component from entering thesecond layer 120. Similarly, it is possible to provide a red filter 142to block the undiffracted green color component and the residualundiffracted blue color component, if any, from entering the third layer110. The filters 142 and 140 can be color coatings, for example.

The EPE 10, according to the present invention, has at least twodiffractive elements H1, H2 arranged adjacent to each other so that thelight distribution is expanded in one direction by H2. An exemplaryarrangement of the diffractive elements H1 and H2 is shown in FIG. 4. Itis possible to arrange the diffraction gratings 112, 114, 122, 124, 132and 134 such that the exit beam exits the EPE 10 from a differentdirection. Thus, with the same input beam 70, it is possible to have anexit beam 80 or an exit beam 90.

The EPE 10 can be used in a portable device 100, such as a mobile phone,personal digital assistant (PDA), communicator, portable Internetappliance, hand-hand computer, digital video and still camera, wearablecomputer, computer game device, specialized bring-to-the-eye product forviewing and other portable electronic devices. As shown in FIG. 5, theportable device 100 has a housing 210 to a house a communication unit212 for receiving and transmitting signals containing information fromand to an external device (not shown) if communications are needed. Theportable device 100 also has a controlling and processing unit 214 forhandling the received and transmitted information, and a virtual displaysystem 230 for viewing. The virtual display system 230 includes amicro-display or image source 192 and an optical engine 190. Thecontrolling and processing unit 214 is operatively connected to anoptical engine 190 in order to provide image data to the image source192 for displaying an image thereon. The EPE 10, according to thepresent invention, can be optically linked to an optical engine 190which is used to provide an original image, for example. It should benoted that each of the diffractive elements H1 and H2 could be aholographic optical element (HOE) or a diffractive optical element(DOE). As the name suggests, a holographic optical element isholographically produced where at least two coherent light beams areused to produce interference fringes. In contrast, a diffractive opticalelement can be mechanically or chemically produced. The EPE 10 can havetwo or more diffractive elements.

The objective of the present invention is to achieve substantiallyuniform color distribution among different wavelengths in the exit beam.The application for the EPE of the present invention is not limited tovirtual displays. The selective reflection control in a planar waveguide(substrate 60), according to the present invention, can also be used inany application where optical beam expansion in one or more directionsis required and light of different wavelengths is used. The diffractiveelements (H1, H2) are, in fact, optical couplers and light modulatordevices for coupling light into the planar waveguide. Thus, the EPE 10,as shown in FIGS. 3 a–3 d, can be viewed as an optical device comprisedof a planar waveguide and a plurality of optical couplers (or lightmodulator devices) disposed adjacent to or on the waveguide for lightcoupling and manipulating purposes.

The image source 192, as depicted in FIG. 5, can be a sequential colorLCOS (Liquid Crystal On Silicon) device, an OLED (Organic Light EmittingDiode) array, a MEMS (MicroElectro Mechanical System) device or anyother suitable micro-display device operating in transmission,reflection or emission.

Thus, although the invention has been described with respect to apreferred embodiment thereof, it will be understood by those skilled inthe art that the foregoing and various other changes, omissions anddeviations in the form and detail thereof may be made without departingfrom the scope of this invention.

1. An optical device comprising: an exit surface section; an inputsurface section to admit a light beam, the light beam comprising atleast a first wavelength component and a second wavelength component;and at least a first layer and a second layer, each of the first andsecond layers comprising a first diffractive element and a seconddiffractive element, such that at least a part of the admitted lightbeam is diffracted in the first diffractive element in the first layerfor providing a first diffracted portion; at least a part of theadmitted light beam is diffracted in the first diffractive element inthe second layer for providing a second diffracted portion; at least apart of the first diffracted portion is further diffracted in the seconddiffractive element in the first layer, thereby forming a first part ofan exit beam exiting through the exit surface section; and at least apart of the second diffracted portion is further diffracted in thesecond diffractive element in the second layer, thereby forming a secondpart of the exit beam exiting through the exit surface section.
 2. Theoptical device of claim 1, wherein the amount of the first wavelengthcomponent in the first part of the exit beam is greater than the amountof the second wavelength component in the first part of the exit beam,and the amount of the second wavelength component in the second part ofthe exit beam is greater than the amount of the first wavelengthcomponent in the second part of the exit beam.
 3. The optical device ofclaim 1, wherein the admitted light beam further comprises a thirdwavelength component, said optical device further comprising: a thirdlayer comprising a first diffractive element and a second diffractiveelement, such that at least a part of the admitted light beam isdiffracted in the first diffractive element in the third layer forproviding a third diffracted portion, and at least a part of the thirddiffracted portion is further diffracted in the second diffractiveelement in the third layer, thereby forming a third part of the exitbeam exiting through the exit surface section.
 4. The optical device ofclaim 3, wherein the amount of the first wavelength component in thefirst part of the exit beam is greater than the amount of the secondwavelength component in the first part of the exit beam and is alsogreater than the amount of the third wavelength component in the firstpart of the exit beam; the amount of the second wavelength component inthe second part of the exit beam is greater than the amount of the firstwavelength component in the second part of the exit beam and is alsogreater than the amount of the third wavelength component in the secondpart of the exit beam; and the amount of the third wavelength componentin the third part of the exit beam is greater than the amount of thefirst wavelength component in the third part of the exit beam and isalso greater than the amount of the second wavelength component in thethird part of the exit beam.
 5. The optical device of claim 2, furthercomprising a filter disposed between the first diffractive element inthe first layer and the first diffractive element in the second layer soas to reduce the amount of the first wavelength component in the secondpart of the exit beam.
 6. The optical device of claim 4, furthercomprising a filter disposed between the first diffractive element inthe first layer and the first diffractive element in the second layer soas to reduce the amount of the first wavelength component in the secondpart of the exit beam and the amount of the first wavelength componentin the third part of the exit beam.
 7. The optical device of claim 6,further comprising another filter disposed between the first diffractiveelement of the second layer and the first diffractive element of thethird layer so as to reduce the amount of the second wavelengthcomponent in the second part of the exit beam and the amount of thesecond wavelength component in the third part of the exit beam.
 8. Theoptical device of claim 6, further comprising another filter disposedbetween the first diffractive element of the second layer and the firstdiffractive element of the third layer so as to reduce the amount of thefirst and second wavelength components in the second part of the exitbeam and the amount of the first and second wavelength components in thethird part of the exit beam.
 9. The optical device of claim 1, whereinat least one of the first and second diffractive elements in the firstand second layers is a holographic optical element.
 10. The opticaldevice of claim 1, wherein at least one of the first and seconddiffractive elements in the first and second layers is a diffractiveoptical element mechanically or chemically produced.
 11. The opticaldevice of claim 3, wherein at least one of the first and seconddiffractive elements in the first, second and third layers is aholographic optical element.
 12. The optical device of claim 3, whereinat least one of the first and second diffractive elements in the first,second and third layers is a diffractive optical element mechanically orchemically produced.
 13. The optical device of claim 1, wherein thefirst wavelength component has a first wavelength range and the secondwavelength component has a second wavelength range longer than the firstwavelength range.
 14. The optical device of claim 3, wherein the firstwavelength component has a first wavelength range, the second wavelengthcomponent has a second wavelength range longer than the first wavelengthrange, and the third wavelength component has a third wavelength rangegreater than the second wavelength range.
 15. The optical device ofclaim 3, wherein the first wavelength component comprises a blue colorwavelength component, the second wavelength component comprises a greencolor component and the third wavelength component comprises a red colorcomponent.
 16. A method of improving color uniformity in an exit beam inan optical device, the optical device having an input surface section toadmit a light beam, the light beam comprising at least a firstwavelength component and a second wavelength component; and an exitsurface section for allowing the exit beam to exit the optical devicethrough the exit surface, said method comprising: providing at least afirst layer and second layer in the optical device; providing a firstdiffractive element and a second diffractive element on the first layer;providing a first diffractive element and a second diffractive elementon the second layer, such that at least a part of the admitted lightbeam is diffracted in the first diffractive element in the first layerfor providing a first diffracted portion; at least a part of theadmitted light beam is diffracted in the first diffractive element inthe second layer for providing a second diffracted portion; at least apart of the first diffracted portion is further diffracted in the seconddiffractive element in the first layer, thereby forming a first part ofan exit beam exiting through the exit surface section; and at least apart of the second diffracted portion is further diffracted in thesecond diffractive element in the second layer, thereby forming a secondpart of the exit beam exiting through the exit surface section, whereinthe amount of the first wavelength component in the first part of theexit beam is greater than the amount of the second wavelength componentin the first part of the exit beam, and the amount of the secondwavelength component in the second part of the exit beam is greater thanthe amount of the first wavelength component in the second part of theexit beam.
 17. The method of claim 16, further comprising: disposing afilter between the first diffractive element in the first layer and thefirst diffractive element in the second layer so as to reduce the amountof the first wavelength component in the second part of the exit beam.18. The method of claim 16, further comprising providing a third layeradjacent the second layer in the optical device; providing a firstdiffractive element and a second diffractive element in the third layer,such that at least a part of the admitted light beam is also diffractedin the first diffractive element in the third layer for providing athird diffracted portion, and at least a part of the third diffractiveportion is further diffracted in the second diffractive element in thethird layer, thereby forming a third part of the exit beam exitingthrough the exit surface section, wherein the amount of the firstwavelength component in the first part of the exit beam is greater thanthe amount of the second wavelength component in the first part of theexit beam and is also greater than the amount of the third wavelengthcomponent in the first part of the exit beam; the amount of the secondwavelength component in the second part of the exit beam is greater thanthe amount of the first wavelength component in the second part of theexit beam and is also greater than the amount of the third wavelengthcomponent in the second part of the exit beam; and the amount of thethird wavelength component in the third part of the exit beam is greaterthan the amount of the first wavelength component in the third part ofthe exit beam and is also greater than the amount of the secondwavelength component in the third part of the exit beam.
 19. The methodof claim 18, further comprising disposing a filter between the firstdiffractive element in the first layer and the first diffractive elementin the second layer so as to reduce the amount of the first wavelengthcomponent in the second part of the exit beam and the amount of thefirst wavelength component in the third part of the exit beam.
 20. Themethod of claim 19, further comprising disposing another filter betweenthe first diffractive element of the second layer and the firstdiffractive element of the third layer so as to reduce the amount of thesecond wavelength component in the second part of the exit beam and theamount of the second wavelength component in the third part of the exitbeam.
 21. A display module, comprising: an optical engine for receivingimage data; a display device operatively connected to the optical enginefor forming an image based on the image data; and an exit pupilextender, comprising: an exit surface section; an input surface sectionto admit a light beam, the light beam comprising at least a firstwavelength component and a second wavelength component; and at least afirst layer, a second layer and a third layer, each of the first, secondand third layers comprises a first diffractive element and a seconddiffractive element, such that at least a part of the admitted lightbeam is diffracted in the first diffractive element in the first layerfor providing a first diffracted portion; at least a part of theadmitted light beam is diffracted in the first diffractive element inthe second layer for providing a second diffracted portion; at least apart of the admitted light beam is diffracted in the first diffractiveelement in the third layer for providing a third diffracted portion; atleast a part of the first diffracted portion is further diffracted inthe second diffractive element in the first layer, thereby forming afirst part of an exit beam exiting through the exit surface section; atleast a part of the second diffracted portion is further diffracted inthe second diffractive element in the second layer, thereby forming asecond part of the exit beam exiting through the exit surface section;and at least a part of the third diffractive portion is furtherdiffracted in the third diffractive element in the third layer, therebyforming a third part of the exit beam exiting through the exit surfacesection.
 22. The display module of claim 21, wherein the amount of thefirst wavelength component in the first part of the exit beam is greaterthan the amount of the second wavelength component in the first part ofthe exit beam and is also greater than the amount of the thirdwavelength component in the first part of the exit beam; the amount ofthe second wavelength component in the second part of the exit beam isgreater than the amount of the first wavelength component in the secondpart of the exit beam and is also greater than the amount of the thirdwavelength component in the second part of the exit beam; and the amountof the third wavelength component in the third part of the exit beam isgreater than the amount of the first wavelength component in the thirdpart of the exit beam and is also greater than the amount of the secondwavelength component in the third part of the exit beam.
 23. Anelectronic device comprising: a data processing unit; an optical engineoperatively connected to the data processing unit for receiving imagedata from the data processing unit; a display device operativelyconnected to the optical engine for forming an image based on the imagedata; and an exit pupil extender, comprising: an exit surface section;an input surface section to admit a light beam, the light beamcomprising at least a first wavelength component and a second wavelengthcomponent; and at least a first layer, a second layer and a third layer,each of the first, second and third layers comprising a firstdiffractive element and a second diffractive element, such that at leasta part of the admitted light beam is diffracted in the first diffractiveelement in the first layer for providing a first diffracted portion; atleast a part of the admitted light beam is diffracted in the firstdiffractive element in the second layer for providing a seconddiffracted portion; at least a part of the admitted light beam isdiffracted in the first diffractive element in the third layer forproviding a third diffracted portion; at least a part of the firstdiffracted portion is further diffracted in the second diffractiveelement in the first layer, thereby forming a first part of an exit beamexiting through the exit surface section; at least a part of the seconddiffracted portion is further diffracted in the second diffractiveelement in the second layer, thereby forming a second part of the exitbeam exiting through the exit surface section; and at least a part ofthe third diffractive portion is further diffracted in the thirddiffractive element in the third layer, thereby forming a third part ofthe exit beam exiting through the exit surface section.
 24. Theelectronic device of claim 23, wherein the amount of the firstwavelength component in the first part of the exit beam is greater thanthe amount of the second wavelength component in the first part of theexit beam and is also greater than the amount of the third wavelengthcomponent in the first part of the exit beam; the amount of the secondwavelength component in the second part of the exit beam is greater thanthe amount of the first wavelength component in the second part of theexit beam and is also greater than the amount of the third wavelengthcomponent in the second part of the exit beam; and the amount of thethird wavelength component in the third part of the exit beam is greaterthan the amount of the first wavelength component in the third part ofthe exit beam and is also greater than the amount of the secondwavelength component in the third part of the exit beam.
 25. Theelectronic device of claim 24, comprising a computer game device. 26.The electronic device of claim 24, comprising a digital camera.
 27. Theelectronic device of claim 24, further comprising a communication unitfor receiving signals containing information indicative to the imagedata, wherein the data processing unit is operatively connected to thecommunication unit for receiving the information.
 28. The electronicdevice of claim 27, comprising a mobile terminal.